Method of purifying caustic soda



Patented Aug. 23, 1938 UNITED STATES FATE T OF F l E William F. Waldeck, Wadsworth, Ohio, assignor to Pittsburgh Plate Glass Company, a corporation of Pennsylvania No Drawing. Application February 15, 1936, Serial No. 64,162

6 Claims.

This invention relates to the production of pure caustic soda from impure solutions containing substantially 50 per cent of sodium hydroxide. The principal object of the invention is to elimi- "nate all contaminating salts from the solution and to obtain directly caustic soda of commercial strength, that is, approximately 50 per cent. A further object of the invention is to provide a process which obviates the necessity of laborious 10 and expensive operations, primarily that of evaporation of the final solution to the desired concentration.

Caustic soda, although a relatively pure material, as measured by commercial standards, conl5 tains a high percentage of impurities. The natureof the impurities varies with the manner of production of the caustic soda. The lime-soda process yields a caustic soda solution which is saturated with respect to sodium carbonate and rsodium sulphate. The electrolytic process yields a caustic solution saturated with sodium chloride. The caustic solution from either process will containvarying amounts of silica, iron, aluminum, calcium, manganese, niclgel, and other metals ac- %cording to the purity of the initial reagents, but the solution will not be saturated with respect to them.

These impurities are objectionable for various reasons. The compounds with which the caustic ..liquor may be saturated, sodium carbonate,

sodium sulphate and sodium chloride, are sometimes not objectionable in themselves but they may provide a source of trouble in subsequent handlings. For example, a filtered caustic liquor, perfectly clear at the time it is shipped from the plant, may be subjected to temperatures lower than that temperature at which it was filtered. As a result of this decrease in temperature, those compounds with which the caustic liquor is saturated will partially precipitate and render the liquor turbid. Accordingly the liquor must be refiltered before use to remove these precipitated impurities. Sodium chloride is the most objectionable impurity inasmuch as it is present to the extent of about 12 per cent in per cent electrolytic liquor and unless completely removed will affect the value and industrial applicability of the final product. The metallic impurities, iron, aluminum, calcium, manganese, and nickel, while 50".present only in small amounts, are undesirable to the rayon and soap industries, which use a major portion of the caustic soda produced. Therefore, they must be eliminated in order that the caustic solution will comply with the stringent specifica- -"Jtions set forthby those industries. Furthermore,

the caustic solution is generally of a yellowishbrown color, a property which is also detrimental to the final product.

There are, of course, a number of processes in which the purification of caustic soda is attempted. Several are designed to remove partially one or two of the impurities contained in 50 per cent caustic liquor. Others attempt to remove only one of the contaminating materials. It is believed, however, there is no process which will remove, even partially, all of the materials which impair the quality of commercial caustic soda.

Patent No. 1,733,879, issued on October 29, 1929, to A. H. Hooker and W. J. Marsh discloses a method in which the caustic liquor is concentrated to 40 to 46 per cent, cooled, filtered, diluted to 38 per cent, and then cooled to 10 degrees C., to precipitate crystals of NaO-I-I.3.5 H2O. These crystals are then separated from the mother liquor by centrifuging, melted and evaporated to 50 per cent NaOH. It should be readily apparent that this patented process requires a great number of separate operations to obtain the desired results. Furthermore, in the final evaporation to 50 per cent after the purification has been effected, metallic contamination from the metal of the evaporators is inevitable, thereby negating at least a part of the purification. It should also be obvious that the dilution of a caustic liquor with subsequent evaporation is uneconomical due to increased fuel and handling costs.

I have found, and it is the substance of the present invention, that caustic soda may be eifectively and economically obtained in a substantially pure state by the formation of crystals of dihydrated sodium hydroxide, NaOI-LZI-IzO, from a solution containing in addition to the caustic soda high percentages of contaminating salts. The dihydrate, as the dihydrated solid variety of sodium hydroxide will hereinafter be called, contains 52.63 per cent of NaOH by weight.

It may be formed by cooling solutions containing from 42.7 to 51.7 per cent of NaOH to below 12.5 degrees C.

It has also been determined that if ordinary commercial caustic liquors be so treated, wellformed, relatively large crystals of the dihydrate precipitate from the mother liquor in substantially pure form and may be separated from the mother liquor without difficulty. Such crystals are comparatively free of all impurities with which the liquor was not originally saturated. Furthermore, and it is a salient feature of my invention, these crystals contain. a much lower percentage of impurities with which the original liquor was saturated, than did the original liquor. Although theoretically this may seem unfounded, it is readily explained. When a solution is saturated with two substances, for example sodium hydroxide and sodium sulphate, and on cooling the saturated solution one of these substances precipitates as a hydrate of such water content that the residual mother liquor is reduced in amount but not substantially changed in water content, then the second substance, in the instant case, sodium sulphate, will precipitate in proportion to the amount of the first substance, sodium hydroxide, which has been precipitated. Therefore, if 50 per cent of the sodium hydroxide has been precipitated, approximately 50 per cent of the sodium sulphate will also. precipitate. However, in actual amount there is several hundred times as much sodium hydroxide in solution as sodium sulphate, and the dihydrate crystals are relatively large while the sodium sulphate crystals are much smaller in comparison. After crystallization there remains, therefore, a precipitate of large dihydrate crystals and a small amount of sodium sulphate crystals, so fine as to appear only as a turbidity in the mother liquor. This same phenomena is equally true when sodium carbonate and sodium chloride are present in solution.

It has been observed that it is a simple operation to separate the two types of crystals utilizing any of the various known means. Hydroseparation, filtration through a coarse filter, classification in a classifier, centrifuging, washing the crystals with water or pure caustic solution to dissolve out the sulphate, carbonate or chloride crystals, which have a much higher specific surface than the dihydrate crystals, or any other method readily apparent to one skilled in the art will serve to remove the solid impurities. The

preferred method is to treat the suspension of crystals in a centrifuge with a coarsely perforated basket, whose openings are so calculated as to retain the large dihydrate crystals but to permit the fine sulphate, carbonate or chloride crystals to pass through the perforations with the mother liquor.

It is manifest that this method of separating hydrated crystals of caustic soda from crystallized impurities is not limited to dihydrate crystals. I have found that by controlling the crystal growth and conditions under which the various hydrates of caustic soda are produced, they may be obtained of such size compared to the crystallized impurities, that a separation is easily effected. Although the present invention is directed to the 42.7 to 51.7 per cent of NaOH, it is possible to change this range of concentrations by the presence or addition of soluble inorganic or organic substances. As examples of these additional substances, ammonia and ethyl alcohol may be mentioned specifically although it is obvious that many other similar compounds are equally as effective. Further, it is possible to take compositions outside of this range of concentrations which will, upon cooling, first precipitate the monoor 3.5 hydrate, and then on further cooling precipitate the dihydrate, so that the concentration range given is to be understood as in no way limiting the applicability of this invention. In this connection it may be stated that an effective measure to assure the precipitation of the dihydrate is to super-cool the impure solution and then add seed crystals of the dihydrate. In this way the dihydrate is crystallized rather than one of the other hydrate forms.

The advantages of my method of purification over the known methods are manifest. All impurities are substantially completely removed. The product is approximately 52 per cent NaOH, thus requiring no evaporation to form the caustic soda of commercial strength, during which process metal impurities would be collected. No uneconomic dilution is required, as the desired hydrate is precipitated from the original solution. Impurities which precipitate with the dihydrate are of such form that they are easily removed. The amount of refrigeration and the amount of liquid to be handled are kept at a minimum and are much less than in an operation involving a hydrate other than the dihydrate.

What I claim is:

l. A method of treating caustic soda liquors containing approximately 42.7 to 52 per cent of sodium hydroxide and containing sodium chloride in an amount required approximately to saturate the solution, which comprises cooling the solution until a mixture of substantial amounts of relatively coarse crystals of the dihydrate of sodium hydroxide and fine crystals of sodium chloride are precipitated, then removing the mother liquor and finely divided sodium chloride crystals by filtration through a foraminous body having interstices of a size sufiicient to permit passage of the fine crystals of sodium chloride while retaining the larger crystals of sodium hydroxide.

2. A method of treating caustic soda liquors containing approximately 42.7 to 52 per cent of sodium hydroxide and being substantially saturated with at least one impurity selected from a group consisting of sodium chloride, sodium carbonate, sodium sulfate, which are inherent in crude caustic soda liquors and which comprises cooling the solution until a mixture of substantial amounts of relatively coarse crystals of dihydrate of sodium hydroxide and fine crystals of said impurity are precipitated, then removing the mother liquor and finely divided salt crystals by filtration through a foraminous body having interstices of a size sufficient to permit passage of the fine crystals of salt while retaining the larger crystals of sodium hydroxide.

3. A process as defined in claim 2 in which the mixture of coarse and fine crystals is washed with an aqueous medium in order selectively to dissolve the fine crystals.

4. A process of removing dissolved salt from caustic soda liquor of a concentration at least sufiicient to form the heptahydrate, said solution being substantially saturated with a sodium salt, which comprises cooling the solution until a mixture of substantial amounts of fine crystals of said salt and coarse crystals of hydrated sodium hydroxide is formed in the liquor, then filtering the mixture through a foraminous body having interstices of a size sufficient to permit passage of the salt crystals while retaining the crystals of sodium hydroxide.

5. A process as defined in claim 4 in which the crystals of sodium hydroxide are Washed with an aqueous medium selectively to dissolve the salt crystals.

6. A process of preparing purified sodium hydroxide from an aqueous solution of sodium hydroxide from which the dihydrate of said hydroxide may be precipitated, said solution being substantially saturated with a sodium salt which comprises slowly cooling said solution until substantial amounts of relatively coarse dihydrate crystals and relatively fine crystals of said salt are precipitated and separating the mother liquor and the relatively fine crystals from the relatively coarse crystals by filtration through a foraminous body having interstices of a size sufiicient to permit passage of the fine crystals of said salt while retaining the larger dihydrate crystals.

WILLIAM F. WALDECK. 

